Investigation of Fuel Geometry and Solid Fuel Combustion for Solid Fuel Ramjets

Solid fuel ramjets (SFRJs) are a simple means of sustaining supersonic flight. The utilization of solid fuels eliminates the need for moving parts or liquid delivery systems, and the solid fuels are typically inert, resulting in minimal handling requirements compared to solid propellants. Characteristic of SFRJ systems are the relatively high combustor velocities and the required gasification of the solid fuel prior to releasing heat through gas-phase reactions. The primary objectives of the current work were to investigate the decomposition behavior of model solid fuels typically used in SFRJ systems and to employ a novel fuel geometry to increase the flame-holding limits of an SFRJ.

Two bench-scale solid fuel experiments were conducted to capture relevant performance metrics of five solid fuels. Performance parameters such as regression rates, surface temperature variations, molten layer thickness, and condensed-phase kinetic behavior were analyzed using a non-combusting laser pyrolysis experiment. Further investigations were performed for each fuel using a modified counterflow burner, which served as an analog for the boundary layer combustion in an SFRJ by introducing the effects of flame heat feedback to the fuel surface. General trends among the fuels were identified, and several mechanistic differences in the decomposition process were discussed with consideration of condensed-phase behavior. The results from the laser pyrolysis and counterflow burner studies were subsequently used as validation data for the development of a solid fuel decomposition model incorporating single-step decomposition, transient heat transfer, and surface heat losses. The developed model showed reasonable agreement with experimental pyrolysis results, particularly for regression rates and surface temperatures of polymethylmethacrylate (PMMA) and hydroxyl-terminated polybutadiene (HTPB).

Investigations using two lab-scale SFRJs were conducted to determine the feasibility and performance impacts of implementing a cavity-style flame holder as a means of improving the flammability limits of a SFRJ. The results presented demonstrate the effectiveness of such a method showing that introducing a cavity flame holder enables significantly higher fuel loading in the present system. The effects of the alternate geometries on local regression rates are reported and a high local heat flux at the cavity corner is identified as a strong factor in the increased flame holding capability. The increased regression rates contribute to higher observed chamber pressures while the effects on combustion efficiency are observed to be minimal. Further investigation of the cavity geometries using an optically accessible SFRJ allowed the analysis of the reacting flow field. High-speed chemiluminescence, high-speed videography, and high-speed three-color camera pyrometry provided further insight into the reacting flow and identified key reaction regions relevant to flame holding. Observations of the spatial regression rate show similar trends to the initial experiments, revealing a large increase in regression rate associated with the cavity corner. The regression rates and observations regarding the size of the recirculation region were incorporated into a semi-empirical model describing the behavior of the recirculation region and point to the increased fuel flow rate resulting from the cavity corner as a contributing factor in the increased flammability of the cavity fuel grains.